Step 1: Split the pituitary in two.
The pituitary gland has two lobes, an anterior (front) lobe and a posterior (back) lobe. Each releases different hormones.
Step 2: What the anterior lobe makes.
Growth hormone, prolactin, and FSH are all anterior pituitary hormones. So options 1, 2 and 3 belong to the front lobe, not the back.
Step 3: What the posterior lobe does.
The posterior lobe does not make its own hormones. It only stores and releases two that are produced in the hypothalamus, namely oxytocin and ADH.
Step 4: Pick the match.
Out of the four choices, only oxytocin is a posterior pituitary hormone.
Step 5: Conclusion.
Therefore the correct answer is "Oxytocin". \[ \boxed{\text{Oxytocin}} \]